1. Field of the Invention
The present invention relates to a method of producing a liquid crystal emulsion composition whose stability is improved so that the liquid crystal structure is not destroyed by dilution, with ease without applying shear force and without using specific equipment.
Further, the present invention relates to a method of producing a liquid crystal emulsion composition having an excellent stability by using hydrophilic surfactants having specific molecular structures with which specific components are admixed in specific mixing ratios.
2. Description of the Related Art
Conventionally, utilization of a liquid crystal phase for emulsification has been known. Examples of utilizable liquid crystals include lamellar liquid crystals, reverse hexagonal liquid crystals, and cubic liquid crystals and all of them produce characteristic emulsification systems. Among liquid crystal emulsion compositions, the liquid crystal structure with the use of higher alcohols has also been known; however the resulting products are extremely unstable and a base material for external application with the use of higher alcohols alone causes problems such as poor moisture tolerance, deposition of crystals, and reduction in viscosity. In order to ameliorate these problems, for example, the admixing of the higher alcohols with fatty acids-lipophilic nonionic surfactants, hydrophilic surfactants, polyvalent alcohols, and water has been proposed in the production of liquid crystal-type base material for external application (Patent Reference No. 1: Japanese Patent Laid-open No. 287718/1988); however, a process of applying stirring shock with the use of a homomixer, an ultrasonic emulsifier, a Manton Gaulin emulsifier, or the like is essential upon admixing these compounds to obtain the liquid crystals and thus an additional equipment cost is required.
Further, as for liquid crystal compositions, technology with the use of surfactants has been known. For example, complexes having liquid crystal structure with dipolar surfactants and/or semipolar surfactants, higher fatty acids, and water have been known (Patent Reference No. 2: Japanese Patent Laid-open 217633/1996). Further, it has also been known that a liquid crystal emulsion can be obtained with the use of ceramides which are present in a very small amount in the epidermis of mammals, in which cholesterols and fatty acids are admixed with ceramides in an oily agent, after which water-soluble polyvalent alcohol is added and the resulting admixture is mixed in an aqueous medium and then cooled (Patent Reference No. 3: Japanese Patent Laid-open No. 124432/1997). However, stirring shock by a Manton Gaulin has been required also in these known technologies.
Since the liquid crystal composition with the use of higher alcohols is extremely unstable and has many disadvantages as mentioned above, the admixing of oil components which are solid at room temperature, such as paraffin and Vaseline, while reducing the admixing ratio of higher alcohols, or the use of water-soluble polymers such as gum arabic, carboxyvinyl polymers, and hydroxypropyl cellulose has been proposed to solve these disadvantages; however, the resulting liquid crystal compositions are clouded, lose transparency that is unique to liquid crystals or have increased stickiness and lack the clean texture of liquid crystals.
The abovementioned Japanese Patent Laid-open No. 287718/1988 (Patent Reference No. 1) has also disclosed the use of decaglyceryl fatty acid esters, POE addition polyethylene glycol fatty acid esters, or POE addition hydrogenated castor oil as the hydrophilic surfactants in liquid crystal compositions; however, liquid crystals cannot be formed with these material components unless high shear force such as with a Manton Gaulin is applied.
Further, fatty acid glycerides having an HLB of 7 or less have been used as lipophilic surfactants. However, the process of the production is complicated, in which oil phase components and water phase components are separately admixed and heated and then the oil phase components are added to the water phase components to be pretreated using a homomixer, after which high shear force is applied by a Monton Gaulin or Manton Gaulin homogenizer. Since the hydrophobic bonding of the liquid crystals is weak, the liquid crystal structure is destroyed by dilution with water or the like to form white emulsified particles, which results in the formation of poorly stable liquid crystals.
Further, according to Japanese Patent Laid-open No. 212716/2003 (Patent Reference No. 4), an improved liquid crystal emulsion composition using oil components, higher alcohols, higher fatty acids, hydrophilic surfactants, lipophilic surfactants, water-soluble polyvalent alcohols, and water has been proposed. Here, the composition is composed of oil components, higher alcohols, higher fatty acids, nonionic surfactants, water-soluble polyvalent alcohols, and water and the liquid crystals are formed by setting the ratio of the oil components to nonionic surfactants to from 1:1 to 3:1 and the number of carbon atoms of the higher alcohols to from 12 to 24 and applying stirring shock. Application of high shear force using a homomixer and then a Manton Gaulin homogenizer or a Microfluidizer is required also in this technology.
As mentioned above, in conventional methods of producing liquid crystal compositions, processes of the production are complicated and costly since high shear force is applied to form liquid crystal structure and moreover, the resulting liquid crystals are poorly stable since the liquid crystal structure is destroyed by dilution with water or the like to change the liquid crystals from the transparent state to the state of opaque emulsified particles.